Sputtering technique



June 23, 1970 E. H. MA-YER ET AL 1 I SPUTTERING TECHNIQUE Original Filed Dec.

E. H.MAVER R J MOORE United States Patent Int. Cl. C23c 15/00 US. Cl. 204-192 3 Claims ABSTRACT OF THE DISCLOSURE Thin films evidencing a high degree of uniformity over a large area are obtained by cathodic sputtering techniques utilizing a system including an electrically isolated substrate holder wherein the configuration of the Dark Space is closely .controlled.

This application is a division of copending application Ser. No. 418,142, filed Dec. 14, 1964, now Pat. No. 3,395,- 089 of July 10, 1968.

This invention relates to a technique for the deposition of thin films by cathodic sputtering. More particularly, the present invention relates to a technique for the deposition of thin films evidencing uniform sheet resistivities.

In recent years, considerable interest has been generated in thin films and the preparation of such films by cathodic sputtering techniques. Unfortunately, it has frequently been noted that the sheet resistivity of deposited films over the surface of a substrate member is prone to variability and nonuniformity, so precluding simultaneous tolerance adjustments. Heretofore it has generally been found that variations in the parameter of interest for a typical substrate, 2" x 3", range to several percent thereby maximizing the difficulties encountered in the subsequent adjustment technique.

In accordance with the present invention, a technique is described for cathodically sputtering thin films manifesting a high degree of uniformity in electrical properties over a broad area. The inventive procedure involves sputtering in a system including an anode member, a cathode member and an electrically isolated substrate holder, the elements of said system being parallel to each other, so permitting the boundary of the Dark Space to be more readily apparent visually. This in turn permits the operator of the process to adjust the sputtering conditions, specifically the Vacuum in the system, until the outline of the Dark Space becomes flat and parallel to the substrate holder.

The invention will be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

The figure is a front elevational view, partly in section, of an apparatus utilized in the practice of the invention.

With reference now more particularly to the figure, there is shown a vacuum chamber 11 provided with an inlet 12 for introduction of a suitable sputtering gas, an outlet 13 for connection to a vacuum pump (not shown) and a base plate 14 which is maintained at ground potential. Shown disposed within chamber 11 is a substrate holder 15, to which a substrate member 16 is attached by means of clips 17, an anode ring member 18 and a cathode member 19, the latter being comprised of the material which is required to be deposited upon substrate member 16- Cathode member 19 is connected to the negative pole 20 of a direct current high potential supply, the positive pole of which is connected with anode 18, as at 21 and to ground through switch 25, substrate holder 15 being maintained at a floating potential.

The present invention may conveniently be described by reference to an illustrative example wherein it is desired to cathodically sputter any of the well known filmforming metals, for example, tantalum, niobium, titanium, zirconium, aluminum, et cetera, in an apparatus of the type shown in the figure.

The substrate member is first vigorously cleaned and then placed upon substrate holder 15, the latter being composed of, for example, nickel, stainless steel, et cetera.

The vacuum techniques utilized in the practice of the present invention are known (see Vacuum Deposition of Thin Films, L. Holland, I. Wiley & Sons, Inc., New York, 1956). By this process, the vacuum chamber is first evacuated to as low a pressure as the system is capable of attaining, typically less than 1 l0 torr, the sub strate being heated during the pumpdown. Following pumpdown, an inert gas, for example, argon, helium, neon, et cetera, is admitted into the chamber, the inert gas input being controlled so as to dynamically stabilize the chamber pressure at the required sputtering value. The pressure required is dependent upon consideration of several factors which are well known to those skilled in the art. However, for the purposes of the present invention, a practical pressure would be within the range of 5x10 to 15 10' torr.

After the requisite pressure is attained cathode 19 which may be composed of any of the above-noted filmforming metals, or, alternatively, may be covered with any of the film-forming metals, for example, in the form of a foil, is made electrically negative with respect to anode 18 which is isolated from the base plate 14, substrate holder 15 being maintained either electrically isolated or in a grounded state. Thereafter, the pressure in the system is adjusted so that the outline of the Dark Space configuration is flat in nature and parallel to the substrate holder and sputtering of the desired film is effected.

The minimum voltage necessary to produce sputtering is dependent upon the particular film-forming metal employed. For example, a direct current potential of approximately 5000-6500 volts may be employed to produce a sputtered layer of tantalum suitable for the purposes of this invention, minimum voltages for other film-forming metals being well known to those skilled in the art. However, in certain instances it may be desirable to sputter at voltages greater than or less than the noted voltage.

The spacing between the substrate holder, anode and cathode is not critical. However, the minimum separation is that required to produce a glow discharge.

The balancing of the various factors of voltage, pressure and relative positions of the cathode, anode and substrate holder to obtain a high quality deposit is well known in the sputtering art. However, it will be appreciated that the main impact of the present invention lies in the discovery that the use of specific electrode configuration during sputtering permits control of film parameters.

With reference now more particularly to the example under discussion, by employing a proper voltage, pressure and spacing of the various elements Within the vacuum chamber, a layer of a film-forming metal is deposited upon the substrate member, sputtering being conducted for a period of time calculated to produce a film having the desired thickness.

Examples of the present invention are described in detail below. These examples are included merely to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE I This example describes the preparation of tantalum films having a uniformity of sheet resistivity within tol- 3 erances of :1 percent, in an apparatus of the type shown in the figure.

In the apparatus employed, anode 18 was a 16" nickel circular member designed with a 7 /2" x 7 /2" hole in which substrate holder was mounted, holder 15 being electrically isolated from anode 18 and from ground. Cathode 19 was a 14" planar element of .050" thick capacitor grade tantalum, the cathode being spaced approxi mately 3 /2 from the anode. Substrate 16 was a 1 X 3" glass microscope slide previously cleaned by conventional cleansing procedures. The high voltage return from the anode was connected to ground.

The vacuum chamber was initially evacuated to a pressure of the order of 10-" torr, argon being admitted until a dynamic pressure (in the bell jar) of 16x10" torr was obtained.

Following, a direct current voltage of 6500 volts was impressed between the cathode and the anode, so resulting in the formation of the well-known Crookes Dark Space. The pressure was then adjusted by throttling or a change of fiow rate to a pressure of 20 10 torr, so resulting in a Dark Space approximately one-half the inner electrode spacing. Next, the pressure was rapidly decreased to 13 10 torr at which time a dome shape was observed over the substrate holder. Next, the pressure was increased to 16X 10* torr, thereby causing the Dark Space to become fiat and parallel to the substrate holder. Sputtering was then continued at a deposition rate of 200 Angstroms per minute until a film of 1200 Angstroms in thickness was produced. The resultant tantalum film evidenced a sheet resistance of l5.00- -0.1 ohms per square throughout the 6" x 6" surface area of substrate material.

EXAMPLE II The procedure of Example I was repeated nine times. The resultant tantalum films evidenced an average sheet resistance distribution of less than *-O.8 percent.

While the invention has been described in detail in the foregoing specification and the drawing similarly illustrates the same, the aforesaid is by way of illustration 4 only and is not restrictive in character. The several modifications which Will readily suggest themselves to persons skilled in the art are all considered within the scope of this invention, reference being had to the appended claims.

What is claimed is:

1. A method for controlling uniformity in electrical properties in the deposition of a thin film of a metal selected from the group consisting of tantalum, niobium, titanium, zirconium, and aluminium upon a substrate member by cathodic sputtering which comprises the steps of evacuating a vacuum chamber in which there is deposited a cathode member, an anode member and a substrate holder having a substrate member positioned thereon, the said substrate holder being electrically isolated from said anode and cathode members, said members being substantially aligned parallel to each other, the voltage return from the anode being connected to ground, applying an electric potential across the said anode and cathode members, so resulting in the formation of a glow discharge including Crookes Dark Space, adjusting the pressure in said chamber until the configuration of said Dark Space becomes flat and parallel to said substrate holder and said substrate member, and depositing said thin film upon said substrate member.

2. A method in accordance with the procedure of claim 1 wherein said anode member is electrically connected to a conductive wall of said vacuum chamber.

3. A method in accordance with claim 1 wherein said film comprises tantalum.

References Cited UNITED STATES PATENTS 3,278,407 10/1966 Kay 204192 3,258,413 6/1966 Pendergast 204192 3,139,396 6/1964 Sinclair 204-192 3,121,852 2/1964 Boyd et al 204-192 2,239,642 4/1941 Burkhardt et a1. 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner 

